Metering pump aggregates (pump units) are known to be designed as positive-displacement pumps, which have a metering chamber with a positive-displacement body, for example in the form of a membrane. The problem when conveying degassing media is that gas bubbles can form in the metering chamber. This holds true in particular when the pump is shut down. These gas bubbles detract from metering accuracy, since the gas bubbles take up a portion of the volume of liquid to be metered, and are also compressible. In light of this fact, it is desirable to remove such gas bubbles from the metering chamber as quickly as possible. However, this cannot always be accomplished, and larger quantities of gas can accumulate in the metering chamber, in particular when the pump is shut down. At a certain air/liquid ratio, the pump can then be blocked, meaning that the metering pump can no longer aspirate any liquid.